Maximizing Chemical Growth Retardants
By Roger C. Styer

Getting the most out of your growth regulators requires familiarity with the chemical, appropriate application methods and in-house experimentation.

This is the season when every grower starts worrying abouthow to control plant growth, whether plugs, bedding flats, pots or baskets. Allgrowers should be doing whatever they can to control temperature, provide morelight, reduce humidity, practice good moisture management and use fertilizerslower in NH4 and phosphorus to help control shoot growth and improve rootgrowth. However, even these methods have their limitations. So growers need touse chemical growth retardants to produce quality plants that can be held inthe greenhouse if needed, shipped properly, look appealing in the stores andgrow out well for the end consumer.

Chemical Growth Retardants

The types of chemical growth retardants we use areanti-gibberellic acid (GA) compounds, meaning they work against production ofGA in the plant, which causes stretch. The only growth retardant not classifiedas an anti-GA compound is Florel, which is an ethylene-generating compound andworks differently in the plant. I extensively covered Florel in the October2002 issue of GPN and will not review it here, but if you have questions aboutits use, please refer back to this issue. In the United States, familiar growthretardants include B-Nine, Cycocel, A-Rest, Bonzi and Sumagic. These compoundswork on different parts in the production of GA and can be utilized in tankmixes to get synergistic effects.

B-Nine is the most common and easiest to use of all growthretardants. It works on a wide range of plants, is easy to spray (spray torun-off) and has no long-lasting effects. B-Nine works better in coolerweather, as plants tend to quickly outgrow its control in warmer weather.B-Nine gets into the plant through leaves and stems but has no activity in themedia through the roots. However, this chemical has a few quirks you need toknow about. First, B-Nine takes a long time to get into the plant, and leavesshould stay wet with the chemical for 3-4 hours to get most of it in. Second,B-Nine should not be applied to plants seven days before or after sprayingcopper fungicides, as a phytotoxicity problem may show up. Third, multipleapplications of B-Nine at high rates may delay flowering and make the firstflower size smaller than desired. B-Nine can be tank-mixed with other growthretardants, thereby reducing the flowering delay (See Figure 1, below).

Cycocel also has a broad label for use on many plants. Thischemical is most effective as a spray, getting in through the stems and leaves.Cycocel does have some activity in the media through the roots, but rates forthis purpose are high and not cost-effective. Cycocel should be sprayed toglistening, as too much chemical could cause more leaf yellowing, known ashaloing. However, plants grow out of this problem and cover up the damagedleaves. Just like B-Nine, Cycocel should stay wet on the plant 3-4 hours.Cycocel works best when tank-mixed with B-Nine (See Figure 1, below).

A-Rest is a stronger chemical than B-Nine or Cycocel and islabeled for a wide range of crops. Used most commonly in plug production,A-Rest also works well on finished crops such as pansies, vinca, salvia,dianthus, lilies and snaps. This chemical gets in through all parts of theplant, making it very useful as a drench. A-Rest gets in very quickly, within30 minutes, so plants do not have to stay wet for a long time as with B-Nineand Cycocel. The main complaint with A-Rest is the cost, but it Á issafer to use than Bonzi or Sumagic, which makes A-Rest a good choice forgrowers who want more control than B-Nine or Cycocel but are afraid of usingBonzi or Sumagic. A-Rest can also be tank-mixed with B-Nine, thereby reducingthe cost per application (See Figure 1, below).

Bonzi and Sumagic work the same way in plants. Bothchemicals work on a wide range of plants, get in quickly like A-Rest but havelonger-lasting effects on plant growth. They get in through stems and rootsonly, not leaves. This means growers have to watch their spray applicationsclosely, making sure they get thorough coverage of stems, but also making surenot to get too much into the soil. Bonzi and Sumagic are very effective in themedia through the roots and can have the most control this way, making themgood choices for drench applications. Growers should be aware of cooltemperatures around the time of application, as more growth control thandesired could occur. These two chemicals are the hardest for growers to apply,making it necessary to standardize the methods between applicators and to conductyour own trials. Both Bonzi and Sumagic can be tank-mixed with B-Nine to avoidoverdosing plants (See Figure 1, page 8).

Application Methods

There are three main ways of applying chemical growthretardants: sprays, sprenches (or media sprays) and drenches. Sprays are themost commonly used method. All of the above chemicals can be sprayed on a widerange of crops. However, Bonzi and Sumagic take more practice by growers, asstem coverage is essential. Because these two chemicals are so active in theplant, it is very easy to get non-uniform coverage, overlap or drift onto cropsyou did not want to spray, such as begonias. B-Nine and Cycocel should beapplied at day’s end, to allow plants to stay wet long enough to get most ofthe chemical inside. Incorrect rates or application with Cycocel can result inconsiderable leaf haloing, making a crop unsaleable until those damaged leavesare covered.

Growers familiar with using Florel understand the importanceof water pH and chemical activity. However, water pH is also important for theother chemical growth retardants. Make sure your water pH in the spray tank is5.5-6.5 for best effectiveness. Rinse spray tanks thoroughly after using,particularly when using Bonzi or Sumagic. Some growers keep separate spraytanks for Bonzi and Sumagic.

Residue problems occur with A-Rest, Bonzi and Sumagic. Thisis due mainly to high spray rates getting onto the plastic trays, benches andconcrete floors. The residue can stay there a long time and is not easilywashed off. Growing a sensitive crop such as begonias right after growingpetunias that have been sprayed with Bonzi in the same area can result in somestunting. To effectively remove residues, soak containers or surface areas forone hour with Greenshield solution and rinse off. Many growers will not reuseflats or pots that may have residue problems for crops that are sensitive tothose chemicals. A similar situation occurs with growers using their owncompost. Bonzi and Sumagic stay in plants a long time and will still be activein compost.

A relatively new technique for growth retardant applicationis a “sprench.” This involves spraying the chemical at volumes higherthan those used for plant sprays and spraying the soil surface when plants arevery small. Normally, growth retardants are sprayed at 2 quarts per 100 sq.ft.of bench space. With sprenches, chemicals such as A-Rest, Bonzi and Sumagic aresprayed at 4 quarts per 100 sq.ft. or slightly higher. Rates used are lowerthan spray rates, as the goal is to get the chemical into the soil to be activethrough early rooting, either in plugs or finished containers. In plugproduction, sprenches keep crops from stretching very quickly duringgermination and can reduce the number of subsequent sprays needed for heightcontrol. Sprench Á rates for A-Rest and Bonzi used on plugs, whichshould be applied before germination is completed, range from 1⁄2-5 ppm,and half as much for Sumagic. For sprenching finished flats right aftertransplanting, double the above rates. A “media spray” can be doneeither right before or after transplanting, using normal spray rates andvolumes. Remember, a sprench uses a greater volume, so reduce the ppmaccordingly.

More and more growers are learning how to drench finished beddingplants. A drench uses a higher volume but lower ppm compared to a sprench.Typically, a drench is applied at 8 quarts per 100 sq.ft., with rates half asmuch as a sprench. Drenches are used with A-Rest, Bonzi or Sumagic, all ofwhich are active in the media through the roots. Drenches can be applied toplugs to hold them for transplanting, to vigorous Á

vegetative liners before potting into combos, to finishedbedding plants when up to saleable size and starting to flower, and to potcrops to slow down growth and hold them for sale. Growers can apply drenches byhand through a portable injector, through a drip system or throughebb-and-flood systems. It is important to calibrate how much active ingredientyou are putting into the container and standardize the method between growers.

There are some important facts to remember when applyingdrenches: 1) Make sure containers have been watered the day previously, as drysoil ties up the chemical; 2) work out your volume and ppm used through trialsand duplicate the method each time; 3) apply drenches when plants are up tosaleable size and starting to flower; 4) make sure plants grow out in 2-3weeks; and 5) pine bark in the media will tie up some of the chemical. Plantscan be sprayed effectively before flowering, but Bonzi and Sumagic can delayflowering if buds are showing when sprayed. Drenches do not delay flowering.

The goal of a drench is to hold the crop for 2-3 weeks, thenlet plants grow out of the chemical. You can reapply to hold them longer. Thistechnique is very effective when spring sales are late. Increase ppm used by50-100 percent when pine bark is in the media, depending on the amount of barkin the mix and how composted it is.

When applying a drench to bedding flats and 4-inch pots notspaced, use a portable injector and drench the plants like you were feedingthem. You will use less chemical if the crop was watered previously and mediais moist before drenching. Reduce the ppm used, as you are putting on morevolume than a standard drench where you know how much volume was applied andwhat ppm was used. When drenching larger pots or hanging baskets, you shouldadd a known amount of solution to each and every pot. When drenching through adrip system, calibrate how much solution comes out at the beginning and end ofthe lines. The longer the drip line, the more variation you will see. Whendrenching with ebb-and-flood systems, cut the rates to 1⁄4 to 1⁄2of normal top-down drench rates. This is due to more chemical getting to theroots from the bottom up. If using water saucers, consider rates to be closerto flood floors.

Vigorous vegetative liners can be drenched (or dipped) withA-Rest, Bonzi or Sumagic before they are planted into combos. This techniquewill hold back growth for 2-3 weeks, allowing the slower plants in the combo tostart growing. Once the vigorous plants come out of the growth regulator, theywill fill in the combo without overgrowing the slower crops. Vigorous linersinclude vegetative petunia, sweet potato vine, verbena and some calibrachoasand helichrysum. Use 2-5 ppm of Bonzi or Sumagic, higher for A-Rest.

Starting Points

To get started with spraying bedding plants, consider thefollowingrates and adjustaccording to reaction, variety, weather conditions, growing techniques andspray methods:

If spraying plug crops, reduce starting rates by one half.Growers in the North should start with low end of rates. Figure 2, page 10,gives you crop groupings for spraying Bonzi and Sumagic. It is important tounderstand that you cannot use the same ppm for all crops when spraying Bonzior Sumagic. How you spray a crop (volume used) will also determine the rateused.

To get started with drenching finished bedding plants,consider the starting points for rates on different crops in Figure 3, pages11-13. These rates are more for mid-South growers in peat-based media, soNorthern growers will need to cut the rates by half, while far-South growersmay need to increase the rates. Remember, you will naturally need to increaseyour drench rates as the temperature gets warmer and plants grow faster. Orincrease rates if using pine bark in your media.

When learning how to drench crops, you must know not onlyyour technique and the type of growth retardant used but also the vigor of thevarieties, growing environment, fertilizer used and how much time beforeshipping. It is imperative you do some trials to work out your technique and tosee how long it takes the plants to grow out. Remember, you want the plants togrow out of the drench in 2-3 weeks.

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